1. Field of the Invention
The present invention relates to an air conditioning apparatus and more particularly to an air conditioning apparatus including a control device for controlling the operation of a sensor used for detecting temperature.
2. Description of the Related Art
An air conditioning apparatus has inner and external heat exchangers which are part of a refrigerating circuit. The air conditioning apparatus is operable both in heating and cooling modes by reversing a flow of refrigerant in the refrigirating circuit. The air conditioning apparatus works in the cooling mode if the inner heat exchanger functions as an evaporator while the external heat exchanger functions as a condenser. Similarly, the air conditioning apparatus works in the heating mode if the inner heat exchanger functions as a condenser while the external heat exchanger functions as an evaporator.
It is known to use various sensors for detecting temperature, humidity or other indexes in the air conditioning apparatus. Usually, a control device is included in the air conditioning apparatus. The control device, which has a microcomputer, is coupled to the sensors to establish thermally comfortable conditions in response to the detecting results of the sensors. An air conditioning apparatus of this type is disclosed in Japanese Patent Disclosure (kokoku) No.59-16178 (Apr. 13, 1984).
A thermistor, which is known to be a resistive circuit component having a high negative temperature coefficient of resistance is widely used to detect temperature. Resistance between two electrodes or terminals of the thermistor decreases as the temperature increases.
In the air conditioning apparatus, a thermistor is disposed adjacent to the heat exchanger for detecting temperature of refrigerant flowing therein. When the heat exchanger operating as an evaporator, dew is deposited each on the surface of the heat exchanger and on the surface of the thermistor. The thermistor is usually molded with a plastic materials as a protective seal. Nevertheless, the dew on the thermistor is penetrates inside the thermistor through the protective seal and reaches to the electrodes the thermistor. The electrodes of the thermistor are formed with silver or metals containing silver, which often causes undesirable effects known as migration of silver. When the migration is occurred, whiskers of silver in the electrodes gradually grow and extend from one electrode to the other electrode. The whiskers then short-circuit the electrodes during the operation of the thermistor. The short-circuit state is partial and tentative in the beginning of the migration, The short state is resolved as long as the whiskers is thin enough to be melt by the short circuit current. The conductivity between electrodes are varied by the development of the whiskers, which results in shifting the temperature coefficient of resistance of the thermistor. It means that the thermistor indicates higher temperature than the actual temperature. The thermistor finally becomes malfunction because of the whiskers, which now causes the fully short-circuit state between the electrodes.
In a conventional air conditioning apparatus, pulses having a pulse width of 50 msec is periodically (20 sec) applied to the thermistor in place of a fixed D.C. potential for avoiding or delaying the migration.
However, there is a problem because the thermistor is not sensitive to a change in temperature due to the long pulse interval. If the thermistor is not sensitive to a change in temperature, particularly when the heat exchanger functions as a condenser, it would fail to take an appropriate action in a short time against the abrupt or abnormal increase of the temperature of the heat exchanger. Further, if the air conditioning apparatus is a type of an air conditioner which has a control device for delaying a fan to start blowing heated air to a room until temperature of the inner heat exchanger becomes predetermined temperature, it would take a time to blow the heated air even the inner heat exchanger becomes the predetermined temperature.